Demarcation of the role of a double bassist and a percussionist in tandem improvisation unites these two prime slices of up-to-date Free Improv. For the Jazz traditionalist the idea of an entire CD featuring bass and drums playing without another instrument is disturbing enough; that the three tracks on Bestiaries and 10 on Duo are all originals should add to this unease. Listeners entranced by boundary-stretching interface however should be drawn to these sessions.

Already established as one of the best rhythm sections in the Jazz world are Polish twin brothers, bassist Marcin Oleś and drummer Bartłomiej Brat Oleś. Together they have backed a variety of soloists including German clarinetists Rudi Mahall and Theo Jörgensmann and French trumpeter Jean-Luc Cappozzo. Not content with the hard work establishing themselves as Poland’s answer to Paul Chambers and Philly Joe Jones, this CD expands their Jazz roots, demonstrating how tropes developed for a rhythmic-centred music can be extended into more abstract areas without losing their essences.

More nonrepresentational in their playing are drummer Patrick Farmer and bassist Dominic Lash. London-based, Lash is one of the British capital’s in-demand small group bassists, working in the London Improvisers Orchestra, bass saxophonist Tony Bevan’s trio and the Anglo-American Convergence Quartet with cornetist Taylor Ho Bynum, drummer Harris Eisenstadt and pianist Alexander Hawkins. Farmer is a drummer and drum instructor who is in the trio Loris with zitherist Sarah Hughes and electronic manipulator/turntabalist Daniel Jones.

Unencumbered by any Free Jazz references – the cynical may even think musical ones as well – Farmer and Lash produce defiantly lower-case improv where the audible textures produced seem to be as much aleatory as acoustic. Chafing, scouring, bumping and rattling instrumental timbres, some of the crackling pulses replicate the tones produced by electronic instruments. With spanked strings and thumps and rubs from drum tops the results are subdued but have a forceful vividness about them.

Among the intonations produced by Lash are those which have reed-like flutters. Others are created with hand-heel thumps against the strings or tremolo patterns and scrubs that vibrate just below the scroll, as the strings are being craftily tightened and loosened. Specific bass timbres that range from col legno echoes to what sounds like the stroke of a match against a flint don’t exist in isolation either. On “Pard”, the lengthiest track and CD centerpiece, individual discordant parameters are meshed among extended pauses to produce a distinct polyphony. Polyrhythmic and with varying pitch, the effect is blurry and watery at junctures, crisp and tart elsewhere.

Sometimes Farmer seems to be dragging parts of his kits along an unyielding surface; other times he exposes ruffs, drum stick nerve beats, raps on the instrument’s wooden shells and flanged side rims; adds tension to the lugs, plus tightens and loosens head pressure. Rubs, crackles and squeaks arise from Lash’s strings, with the resulting drones attaining ostinato status to sound alongside the drummer’s pummels and whacks.

Lyrical and legato when compared to the Lash-Farmer episodes, the Oleś tracks have a different aim. Bestiaries deals with the liberation of sound, destroying the idea of background or foreground. In contrast the duo moves beyond solipsism to reaffirm that a double bass and drums can constitute a satisfying improvisational unit.

To this end the brothers create tracks such as “White Rainbow”, with faultless rhythm that could serve as the exemplar of how a Jazz rhythm section should sound. At the same time while the bassist and drummer expose linear parallel beats, each is extending himself individually. Marcin’s tremolo shuffle bowing succeeds his thick stopping while Brat decorates his chromatic thrusts with cymbal strokes, flams and rebounds until both lines intersect.

Meanwhile a piece such as “Three chords” utilizes every iota of those chords as Brat’s shattering cymbal work, pops and clatter underlie Marcin’s connective four-string oscillations so well that the combination of his alternating low- and high-pitched dynamics conjure up memories of an Eastern European string ensemble. Working in double counterpoint elsewhere, as on “Skrik”, Marcin’s amplified tremolo lines and flying staccato leaps define his territory while his brother’s puttering beat bounces appropriately without becoming overpoweringly thick. Drum stick pressure along a cymbal top and closely packed sul ponticello string runs are some of the other extended techniques the Oleś brothers use on the CD. Their CD, like the other, is similarly polyrhythmic and other-directed.

The only problematic track is the oddly titled and potentially offensive “Jewisher”. Is the improvisation with its overlay of tremolo melancholy communicated by powerfully outlined bass stopping, resonating cymbal crashes and rattles supposed to represent music more Jewish or “Jewisher” than Jewish? The theme may be highly rhythmic, enlivened with rebounds and ratamacues from Brat, but it scarcely seems invested with Yiddishkeit. This is a puzzle that should be solved.

Putting aside this potential misstep in translation for a time, both duos prove that a band made up of just a bassist and a drummer can create notable sounds on its own.